The section continues to provide expert physical and technological support for radiation treatment, consisting of routine calibration, quality assurance and first-line maintenance of all radiation producing and ancillary equipment. It includes special dosimetry studies, computer- assisted treatment planning, patient monitoring and the design and development of special equipment tailored to developing clinical needs. Diodes are now in routine clinical use for the verification of patient doses and exhibit an accuract of +/- 3. The radiosurgery system has been installed and commissioned. To this date, we have successfully treated 25 stereotactic radiosurgery patients and plan to migrate from single fraction to multi fraction radiosurgery utilizing implantable frame attachment points. The project in collaboration with the Naval Research Laboratory to evaluate the clinical utility of semiconductor doped Vycor glass is well under way. We are continuing the characterization of the dosimetric properties and using these data to refine detector development. Clinical trials are expected to start by early Winter, 1999. The mini multileaf collimator has been commissioned and clinical use will commence shortly. Ongoing projects are the commisioning of a refined stereotactic treatment planning system capable of treating more complex tumor volumes and the development of intensity modulated radiation therapy (IMRT). Sequence driven IMRT is awaiting the delivery of a new Record-and Verify system, which has been ordered. The establishment of an IMRT research program at NCI will include the development of a frameless stereotactic localization system.